1. Field of the Invention
The present invention relates to an AC power supply apparatus for supplying a load with an AC power and Fourier transforming a load current to monitor a spectrum and the like, and to a load current measurement method in an AC power supply apparatus.
2. Description of the Prior Art
Recently, with the diversification of load types connected to an AC power supply apparatus, it has been increasingly required to measure harmonic components contained in a load current.
Heretofore, to achieve such measurement, as shown in FIG. 1, a current value detected by a current detection circuit DET is inputted to an FFT (fast Fourier transformation) analyzer FAZ.
Since the prior art FFT analyzer shown in FIG. 1 is provided separately from an AC power supply PS, it has a so-called window processing is required.
The necessity for the window processing will be described with reference to FIGS. 2-4.
As well known, in analyzing an input signal by the FFT analyzer, the input signal is regarded as a periodical signal having a certain frame time so as to determine its Fourier series. The frame time is a value determined by the design of the analyzer, and is not varied with the input signal. For example, when a sinusoidal input signal shown in FIG. (A) is obtained in the frame time, a problem arises in that a Fourier series of a repeated signal as shown in FIG. 2B is determined.
Therefore, a discontinuous point is generated at a point for connecting the beginning and the end of the frame time, and a spectrum including a leakage (a spectrum number of harmonics) in addition to the proper sinusoidal wave spectrum is determined. However, in an exceptional case, when, as shown in FIG. 3A, the period of the sinusoidal wave exactly fits in one frame (the frame time is an integer multiple of the input signal period), or when, as shown in FIG. 3B, the beginning and the end of the signal are considered to converge to a same value, such discontinuous points are not generated and the exact spectrum can be determined.
Then, as a measure for such a problem, it is generally performed that an original signal is multiplied by a function called a window for reducing the leakage to transform the shape of the original input signal into the shape of the signal whose beginning and end converge at zero, thereby eliminating the generation of discontinuity. For example, when the original input signal is as shown in FIG. 4A, the signal multiplied by the window function is as shown in FIG. 4B (see, also FIG. 14A). The FFT analyzer determines a spectrum according to the window processed signal.
As described above, heretofore, in analyzing harmonic components contained in an AC load current, an appropriate window processing has been required to obtain an exact measurement value.